Floriculture and Ornamental Nurseries

Pythium Root Rot

Pathogen: Pythium spp.

Pythium attacks juvenile
tissues such as the root tip. After gaining entrance to the root the fungus may
cause a rapid, black rot of the entire primary root and may even move up into
the stem tissue. As the soil dries, new roots may be produced and the plant may
recover or never show symptoms of disease. Under
wet conditions brought about by poor soil drainage or excess irrigation, more and more roots are killed and the
plant may wilt, stop growing, or even collapse
and die. Bulbs
of susceptible plants turn black, gradually desiccate, and form a hard mummy.

The pathogens that are responsible for
Pythium root rot, also known as water mold, are present in practically all cultivated
soils and attack plant roots under wet conditions. These fungi can be spread by
fungus gnats and shore flies. There are many species of Pythium; a few of these species are beneficial in that they compete with or parasitize
the pathogenic species. Of the many pathogenic species, some have limited host
ranges while others, such as Pythium ultimum, have very wide host ranges. Some Pythium species, such as P. aphanidermatum, are pathogens only at high temperatures (above 77°F),
and some are active only at low soil temperatures. Soil moisture conditions of
70% or higher are conducive to infection by Pythium. Soil from a given field may contain several
pathogenic Pythium species.

Pythium speciesform several types of spores but not all species form
all types. Zoospores, which are produced in sporangia, are motile in water.
Oospores, which result from a sexual process, usually undergo a period of
dormancy and can withstand long periods of drying. Some species also form chlamydospores,
which are asexual and have thick cell walls. These structures can serve as
resting structures. Sporangia and zoospores in general do not survive in air or
dry soil for long periods of time. ELISA test kits are available for detecting Pythium.

In the control of Pythium diseases, emphasis is placed on providing
good drainage and water management. Steam (at 140°F for 30 minutes), solarize
(double-tent at 160°F for 30 minutes or 140°F at 1 hour), or
chemically treat growing medium. Sanitation is important because Pythium spp. can survive in dust, planting medium, or soil
particles on greenhouse floors and in flats and pots. Remove and discard
diseased plants. Use of properly composted pine bark at 20% in a potting
mixture is reported to provide some control of Pythium and Phytophthora root
rots; also the mycoparasite, Gliocladium virens, is used as a Pythium biocontrol agent. For flower production in open
fields, solarization in warmer climates has been successful for control of
damping off in many crops. Reports
of inadequate control of some high temperature species (e.g. P.
aphanidermatum) have been made. Solarization and steaming are
acceptable for organic production.

Common name

Amount to Use

R.E.I.+

(trade name)

(hours)

When choosing a fungicide, consider the general properties
of the fungicide as well as information relating to environmental impact.

A.

MEFENOXAM

(Subdue Maxx)

Label rates

48

MODE OF ACTION GROUP NAME (NUMBER1): Phenylamide (4)

COMMENTS: The granular formulation may be applied preplant or
the liquid formulation can be applied as a drench at planting. Mefenoxam is
water-soluble and readily leached from soil. It is absorbed primarily through roots and may be translocated in the plant through the xylem.

B.

FOSETYL-AL

(Aliette)

2.5–5 lb/100 gal water for foliar spray

12

MODE OF ACTION GROUP NAME (NUMBER1): Phosphonate (33)

COMMENTS: When applied as a foliar spray, it is absorbed by
foliage and moves into roots. Soil drench is less effective than a foliar application.

C.

GLIOCLADIUM VIRENS#

(SoilGard) 12G

Label rates

NA

1

Group numbers are assigned by the Fungicide Resistance
Action Committee (FRAC) according to different modes of actions (for more
information, see http://www.frac.info/). Fungicides with a different group
number are suitable to alternate in a resistance management program. In
California, make no more than one application of fungicides with mode of
action Group numbers 1, 4, 9, 11, or 17 before rotating to a fungicide with a
different mode of action Group number; for fungicides with other Group
numbers, make no more than two consecutive applications before rotating to fungicide with a different mode of action Group number.

#

Acceptable for use on organically grown ornamentals.

+

Restricted entry interval (R.E.I.)
is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing.

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